1. Field of the Invention
The invention relates generally to a method of forming an epi-channel in a semiconductor device. More particularly, the invention relates to a method of forming an epi-channel in a semiconductor device wherein the epi-channel has a uniform shape. Still more particularly, the invention relates to a method of forming an epi-channel in a semiconductor device that avoids the etching or deterioration of the insulating structure in the surface of a semiconductor substrate.
2. Description of the Prior Art
In a CMOS structure, an epitaxial channel forming process utilizes a SEG process to form a channel below a gate oxide in an epi-silicon layer. Advantages of the epi-channel process include increase in a drain saturation current and reduction in a short channel effect. Also, as the gate oxide is formed of epi-silicon, the physical property of the gate oxide can be improved. Applying the general SEG process includes several problems. In an ESD process using SEG, H2 bake is used as an in-situ cleaning. The purpose of using the H2 bake is to remove oxide on the silicon surface or etch away defects.
Generally, the surface cleaning effect will improve by increasing the temperature and lowering the pressure. Therefore, the SEG process using a LPCVD method must perform the H2 bake at a temperature of more than 900xc2x0 C. and under a pressure of below 20 Torr. The H2 bake at the temperature of more than 900xc2x0 C. and below the pressure of 20 Torr acts to clean the silicon surface, but Vt ion-implanted (i.e., channel ion-implanted) ions are diffused into the substrate. Therefore, not only Vt of the device is made irregular but also an insulating material of the device is etched to remove the insulating material formed at the active silicon interface. Etching of the insulting oxide causes an epi-growth at the active side when the epi-channel process is performed, resulting in an irregular shape of the epi channel. Thus, subsequent processes are difficult to conduct and the irregular shape of the epi-channel deteriorates the electrical property of the device.
A method of forming an epi-channel in a conventional semiconductor device will be now explained by reference to the accompanying drawings. FIGS. 1A to 1C are cross-sectional views illustrating a prior art method of forming an epi-channel in a conventional semiconductor device.
Referring now to FIG. 1A, the remaining regions except for an active region in which a device will be formed in a semiconductor device 1 are etched to form a trench structure. Then, the trench structure is covered with an insulating material 6. In order to adjust the threshold voltage Vt, ions are implanted into the surface 1a of the semiconductor device 1.
Referring to FIG. 1B, in order to remove various oxide films including crystal defects and a native oxide film on the surface of the semiconductor substrate 1, a H2 bake process is implemented. At this time, the H2 bake process is performed at the temperature of more than 900xc2x0 C. and under the pressure of below 20 Torr in order to increase a cleaning effect. Due to the increased temperature and decreased pressure, ions that were implanted into the surface of the semiconductor substrate 1a in order to adjust the threshold voltage are diffused into the inside of the semiconductor substrate 1, thereby changing a desired threshold voltage. Also due to excess cleaning effect, the upper portion 6a of the insulating material 6 contacting the silicon portion of the semiconductor substrate to be grown is etched.
Referring now to FIG. 1C, by means of SEG process, silicon is grown to form an epi-channel 8. At this time, silicon is grown to the etched portion 6a of the insulating material 6, thus making the shape of the epi-channel irregular.
FIG. 2 is a TEM photography showing an epi-channel formed according to a method of forming an epi-channel in a conventional semiconductor device. As shown, the sidewall of the grown epi-channel, that is epi-silicon is formed unevenly.
As a result, as the H2 bake process is performed at high temperature and at low pressure for removing defects on the surface of the semiconductor substrate and for increasing the cleaning effect, the upper portion of the insulating material 6 is undesirably etched, thereby forming an irregularly shaped epi-channel.